When physical methods for separation cannot secure optimal metal value of an ore, leaching is an alternative for extracting precious metals. For most leaching processes, the ore is first prepared. During this first step, the ore is crushed and grinded, to reduce the particle size of the ore and liberate the precious metal(s) for recovery. This may be followed by a gravity recovery process to recover free precious metal particles, such as free gold particles, before being subjected to leaching.
Then, the ore can be subjected to a pre-oxidation (PO) step to oxidize the sulfides and, thereby, limit their interaction with the leaching solution. During the pre-oxidation step, the pulp including ore particles and process water is contacted with an oxidizing agent such as oxygen, hydrogen-peroxide (H2O2) and ferricyanide (Fe(CN)63−).
Following the pre-oxidation step, if any, the ore particles are brought in contact with a leaching solution for some retention time for the leaching solution to react with the particles. For precious metals, such as gold and silver, the leaching solution typically contains cyanide (CN−), a singularly charged anion consisting of one carbon atom and one nitrogen atom joined with a triple bond. During cyanidation, i.e the process of extracting precious metals from an ore, precious metals form complexes with cyanide. Cyanidation is adversely affected by several metal sulfide minerals, such as copper, zinc, and iron, which are generally abundant in precious metal-containing ores. Some of the metal sulfide minerals also dissolve during the cyanidation process. In fact, cyanide will preferentially leach sulfide minerals and react with sulfur to produce thiocyanate, as will be described in more details below. These unwanted reactions consume a significant amount of cyanide and oxygen, which is required for the oxy-cyano-leaching of gold and silver. Leaching kinetics of sulfide minerals are quickly in contact with oxygenated cyanide solutions. Therefore, the solution becomes charged with oxidation products that are mostly metal ions, metallo-cyanide complexes, metal oxides and other sulfur species including thiocyanate, sulfides, polysulfides, thiosulfate, sulfites, sulfates, etc.
The leaching of sulfide minerals containing divalent metals can take place via the following overall reaction (Marsden & House, 2006):2MS+2(x+1)CN−+O2+2H2O2M(CN)x(x-2)-+2SCN−+4OH−
From the reaction above, cyanidation products can be separated into two main categories: metallo-cyanide complexes (i.e., M(CN)x(x-2)-) and sulfur-cyanide complexes (i.e., SCN−).
Formation of thiocyanates (SCN−) reduces the cyanide quantity available for precious metal lixiviation. The formation of thiocyanates is also known to cause an increase in cyanide consumption and, thus in process costs. For other reasons, it is generally desirable to optimize the cyanide consumption during precious metal cyanidation and minimize thiocyanate formation. Furthermore, thiocyanates are highly toxic and cannot be released back to the environment without being treated.
Pre-oxidation of the sulfide minerals could result in the formation of several oxy-sulfur species leading the encapsulated precious metal to be amenable to a cost-effective cyanidation. In recent years, different oxidizing agents have been tested in both pre-oxidation and cyanidation (Bayat, O, Vapur, H, Akyol, F and Poole, C, 2003. Effects of oxidising agents on dissolution of Gumuskoy silver ore. Minerals Engineering, 16: 395-398), (Xie, F and Dresinger, D B, 2009. Use of ferricyanide for gold and silver cyanidation. Transactions of Nonferrous Metals Society of China, 19(3): pp. 714-718), (Breuer, P, Hewitt, D and Meakin, R L, 2008. Does pre-oxidation or lead (II) addition reduce the impact of iron sulfides in cyanidation, Hyrometallurgy 2008: Proceedings of the Sixth International Symposium, 750-757) and (Ellis, S and Senanayake, G, 2004. The effects of dissolved oxygen and cyanide dosage on gold extraction from a pyrrhotite-rich ore. Hydrometallurgy, 72: 39-50). Single or atmospheric oxygen are used as a common source of oxygen. However, their application may not be practically efficient due to their low solubility in the slurry, insufficient aeration, high viscosity of the slurry and strong oxygen-consuming pulps. Alternative oxidants such as hydrogen peroxide, ferricyanide and ozone have also been studied as an oxygen precursor at a laboratory scale (Xie & Dresinger, 2009), (Guzman, L, Segarra, M, Chimenos, J M, Fernandez, M A and Espiell, F, 1999. Gold cyanidation using hydrogen peroxide. Hydrometallurgy, 52: 21-35), (Pedroza, F R C, Aguilara, M J S, Luévanos, A M and Anaya, J A G, 2007. Ozonation Pretreatment of Gold-Silver Pyritic. Ozone: Science & Engineering, 29(4): 307-313).
There is thus a need to reduce thiocyanates accumulation during cyanide-based leaching processes.